1. Field of the Invention
This invention generally relates to coatings for implantable electrodes such as pacing electrodes, neurostimulator electrodes, and electroporating electrodes and sensing electrodes. The coatings are highly biocompatible, having low polarization. They consist of a biocompatible, conductive substrate; a thin film outer layer of biocompatible, conductive carbon; and a biocompatible, conductive intermediate layer having a high surface area. The intermediate layer increases the surface area of the carbonaceous outer layer.
2. Prior Art
Three overriding requirements for implantable electrodes are biocompatibility, biostability, and low energy loss. Broadly, the biocompatibility requirement is met if contact of the electrode with body tissue and blood results in little or no immune response from the body, especially thrombogenicity (clotting) and encapsulation of the electrode with fibrotic tissue. The biostability requirement means that all physical, electrical, and chemical properties of the electrode/coating system remain constant and unchanged over the life of the patient. The low energy loss requirement is met if electrode polarization is a minimum.
In the patent literature, U.S. Pat. No. 4,602,637 to Elmqvist describes a commonly used pacing electrode having sputtered columnar titanium nitride as a coating material. This form of titanium nitride has good conductivity combined with a high specific surface area, resulting in favorable polarization and sensing properties. The disadvantage of titanium nitride, however, is that it degrades the electrical properties of surrounding tissue after implantation. This occurs as the body tissue encapsulates the columnar titanium nitride in fibrotic tissue, which has a lower conductivity than normal tissue.
In the case of a pacing electrode, fibrotic tissue raises the stimulation threshold. The stimulation threshold is the minimum energy required to produce a cardiac contraction. A raised stimulation threshold, in turn, impacts the battery life of the system so that the medical device must be explanted sooner than desired. The encapsulation process also interferes with sensing of intrinsic millivolt signals required by pacemakers. In prior electrode designs, the fibrotic encapsulation problem has been addressed by incorporating a means of metering or eluting steroidal medication to the tissue contact site over time.
However, steroidal medication is not completely effective in eliminating the stimulation threshold rise due to encapsulation. Steroidal medication eluting arrangements have a relatively short duration of effectiveness, and also add cost and complexity to the system, add risk of infection, and, in many cases, a portion of the electrode working surface must be dedicated to the medication administering function.
The problem of encapsulation has also been addressed by the use of carbon as an electrode material. Carbon is much more highly biocompatible than the previously used noble metals and valve metals. For example, U.S. Pat. No. 4,033,357 to Helland et al. describes the use of pyrolitic carbon for pacing electrodes, although no means of increasing the surface area is discussed. In that regard, pacing electrode designs according to this patent meet the requirement of improved biocompatibility, but they do not have the high specific surface characteristics of currently used columnar titanium nitride.
Various methods of reducing the polarization of implantable carbon electrodes by increasing their surface area have also been tried. U.S. Pat. No. 4,281,668 to Richter et al. describes one method comprising heating glassy carbon parts in air to form a porous layer of increased surface area. Gluing the glassy carbon to the electrode stem completed the electrode assembly. Another method is described in U.S. Pat. No. 4,612,100 to Edeling et al. in which high sputter power levels are used to induce porosity on the order of 20 angstroms in sputtered carbon films. U.S. Pat. No. 4,609,508 to Edeling et al. also noted that microscopic porosity is induced in the course of the pyrolytic processing of vitreous carbon. Still more efforts to increase the surface area of carbon thin films in pacing electrodes are described in U.S. Pat. No. 5,074,313 to Dahl et al. This patent teaches sputtering a carbon thin film over a layer of high surface area sputtered metallic aluminum. A similar method is described in U.S. Pat. No. 5,118,400 to Wollam in which metallic titanium, aluminum, or zirconium as high surface area materials are sputtered to a “hillocked” morphology.
In addition to increasing the surface area, U.S. Pat. No. 4,495,039 to Cerise et al. shows that polaraization of pyrocarbon electrodes can be reduced by means of electrochemical activation of the carbon surfaces. This is done by immersing the carbonaceous material in concentrated acid and imposing an electrical current thereto.
Thus, there is still a need for an implantable electrode construction having the requisite improved biocompatibility along with high specific surface characteristics, such as provided by columnar titanium nitride. The present electrode fulfills this need in terms of both low polarization and minimum energy requirements for acceptable sensing properties.